The present invention is related to fluid pressure control valve devices of the proportioning type and particularly to such proportioning type control valve devices as used to graduate the application and release of the air brakes on railway vehicles.
These proportioning type control valve devices are of a basically conventional design in that a diaphragm piston stack comprising a control piston and a compensating piston is carried on a central stem that terminates in an annular exhaust valve seat. Engageable with this exhaust valve seat is a disc valve element that is spring-biased toward engagement with an annular supply valve seat surrounding the exhaust valve seat.
The piston stack is operative to a brake application position in which the exhaust valve seat engages and unseats the disc valve from the supply valve seat in response to a reduction of brake pipe pressure, which acts on one side of the control piston in opposition to a reference pressure on the opposite side. This reference pressure, commonly referred to as control reservoir pressure, is charged from brake pipe pressure via a one-way check valve which traps the pressure at the time of a brake pipe reduction to provide a reference pressure against which the brake pipe reduction is measured.
The resultant pressure differential between brake pipe and control reservoir pressures thus establishes an actuating force differential on the piston stack to move the stack to brake application position. As the resultant flow of supply pressure to the brake cylinders via the unseated supply valve develops, this pressure is conducted also to one face of the compensating piston to effect a force on the piston stack in a direction opposing the actuating force. When these counteracting forces are substantially balanced, the piston stack assumes a lap position in which the supply valve is again seated to terminate any further supply of pressure to the brake cylinders. Since in lap position, the exhaust seat also remains engaged with the valve element, brake cylinder pressure is neither increased nor decreased and is thus maintained constant until the force balance on the piston stack changes, such as to instigate a further brake application or alternatively a brake release.
The desired degree of brake release can be obtained by increasing brake pipe pressure so that the preponderance of forces on the piston stack urge its movement toward brake release position, in which position the exhaust valve seat becomes disengaged from the valve element, which remains seated on the supply valve seat. Consequently, brake cylinder pressure including the pressure acting on the compensating piston is exhausted until the force holding the piston stack in release position in accordance with the degree of brake pipe pressure is counterbalanced by the reduction of pressure on the compensating piston to again obtain a force balance to lap the brakes. In this way, the brake pressure may be increased and released in graduated increments or as a single continuous operation.
Instruction Pamphlet No. 5074-5, published January, 1964 and entitled "27-LB-1 Locomotive Brake Equipment" shows and describes in detail a commercially active control valve device of the above-discussed type and its use in a locomotive brake system.
As is well known in the railroad industry, fail-safe operation is most desirable and continuously strived for. An unintentional loss of brake pipe pressure, for example, will result in an automatic application of the brakes of a locomotive controlled by the proportioning type control valve device described above. Although not as apt to occur as a loss of brake pipe pressure, there does exist the more remote possibility of the reference or control reservoir pressure being lost, which would render the brakes inoperable due to the inability of a brake pipe reduction to establish a pressure differential on the piston stack in a brake application direction.